The present invention relates to digital communication systems and more particularly to systems and methods for exploiting multiple antennas or other channel outputs to exploit spatial diversity and ameliorate the effects of interference.
It is known to use adaptive spatial processing to exploit multiple antenna arrays to increase the communication quality of wireless systems. A weighting among antennas is chosen based on content of the signals received via multiple antenna elements. The spatial processor selects a weighting that optimizes reception of a desired signal.
In some systems, the spatial processor estimates a weighting based in part on statistical characterization of an interference source. The spatial processor selects the weighting to maximize the signal to interference plus noise ratio (SINR). Examples of such systems are described in PCT Pub. No. 98/18271, the contents of which are incorporated herein by reference.
In these systems, the weighting becomes a basis for a maximum likelihood solution for estimating transmitted symbols. The output of the spatial processor is a so-called xe2x80x9chard decisionxe2x80x9d of the most likely transmitted symbol. It would be desirable, however, to combine trellis codes with the use of this type of spatial processing at the receiver because trellis codes provide excellent bit error rate performance with minimal addition of redundant information. A trellis decoder or Viterbi decoder would then be used to remove the effects of the trellis coding. Rather than a xe2x80x9chard decision,xe2x80x9d the trellis or Viterbi decoder requires as input likelihood or so-called xe2x80x9ccost metricxe2x80x9d values corresponding to different values of each transmitted symbol. These cost metric values are also referred to as xe2x80x9csoft decisionxe2x80x9d values. It would be desirable to optimize spatial processing techniques to efficiently provide xe2x80x9csoft decisionxe2x80x9d values as output rather than xe2x80x9chard decisionxe2x80x9d values.
OFDM (Orthogonal Frequency Division Multiplexing) is another highly useful communication technique. In OFDM, the available bandwidth is divided into subchannels that are orthogonal to one another in the frequency domain. A high data rate signal is effectively transmitted as a set of parallel low data rate signals, each one being carried over a separate subchannel. OFDM addresses a problem known as multipath caused by differences in delay time among different paths taken from a transmitter to a receiver. The effect of multipath is intersymbol interference created by energy associated with different symbols sharing a common arrival time. By creating multiple low data rate subchannels, OFDM lengthens the period occupied by a single symbol so that dispersive effects tend to be confined within a single symbol period, thereby reducing intersymbol interference. It would also be desirable to optimize a soft decision output spatial processor to operate in conjunction with OFDM.
A spatial processor that exploits signals that arrive via multiple outputs of a communication channel to provide soft decision values useful to a trellis or Viterbi decoder is provided by virtue of the present invention. A spatial processor according to the present invention may take into account a statistical characterization of interference as received via the multiple channel outputs. Spatial processor operation may also be optimized to operate in conjunction with orthogonal frequency division multiplexing (OFDM) and thereby effectively ameliorate the effects of frequency selective interference.
In accordance with a first aspect of the present invention, a method is provided for receiving an OFDM signal via a plurality of outputs of a channel in the presence of noise and/or interference. The method includes: forming an estimate of a received OFDM frequency domain symbol based on a statistical characterization of noise and/or interference received via the plurality of channel outputs, obtaining a channel confidence level for a frequency subchannel of the OFDM frequency domain symbol, and obtaining cost metric values for various possible values of the received OFDM frequency domain symbol based on the estimate and the channel confidence level.